The present invention relates to a semiconductor memory device for storing image data and, more particularly, to a rewritable nonvolatile semiconductor memory device for storing image data.
A rewritable image data storage system of the following configuration has been known for storing image data. Image data is first obtained by a digitizer, an imaging device, or the like. The data obtained by the digitizer is time-serially supplied to a processor or a vector generator through a data bus. The processor or vector generator processes the digital signal, to produce pixel data. The processor or vector generator then stores the pixel data in a frame buffer. The image data stored in the frame buffer is converted as needed into a luminance signal through reference to a look-up table. The luminance signal from the look-up table and a timing signal generated from a timing generator are supplied to a monitor device, which then displays them as an image. In order to rewrite part of the image data in such a system, the stored content of memory cells in the frame buffer must be rewritten.
With this system, however, an expensive digitizer or imaging device is necessary to obtain the image data, and thus the total cost of the system becomes very high.
When the system is energized, a time-serial signal from the digitizer must be written in the frame buffer. When image data has already been stored on a magnetic disk or the like, the storage data must be read out therefrom and rewritten in the frame buffer The frame buffer often comprises a dynamic RAM. Writing data in the dynamic RAM must be performed outside of the data readout period. For this reason, when a large-capacity dynamic RAM is used, efficiency the data writing in the frame buffer is degraded. For example, when a frame buffer having a display resolution of 1,280.times.1,024 pixels and a refresh rate of 60 Hz comprises a 16-kbit dynamic RAM, the time used for the data write operation is 80% of the total cycle time. When the frame buffer comprises 32 64-kbit dynamic RAMs, the time used for the data write operation is reduced to 30% of the total cycle time.
Even if a dynamic RAM having a very high-speed cycle time is used, it is inefficient for serial image data to be obtained from an external imaging device and written in a memory.